NAME

       File::FcntlLock - File locking with fcntl(2)

SYNOPSIS

         use File::FcntlLock;

         my $fs = new File::FcntlLock;
         $fs->l_type( F_RDLCK );
         $fs->l_whence( SEEK_CUR );
         $fs->l_start( 100 );
         $fs->l_len( 123 );

         open my $fh, '<', 'file_name' or die "Can't open file: $!\n";
         $fs->lock( $fh, F_SETLK )
             or print "Locking failed: " . $fs->error . "\n";
         $fs->l_type( F_UNLCK );
         $fs->lock( $fh, F_SETLK )
             or print "Unlocking failed: " . $fs->error . "\n";

DESCRIPTION

       File locking in Perl is usually done using the flock() function.  Unfortunately, this only
       allows locks on whole files and is often implemented in terms of flock(2), which has some
       shortcomings.

       Using this module file locking via fcntl(2) can be done (obviously, this restricts the use
       of the module to systems that have a fcntl(2) system call). Before a file (or parts of a
       file) can be locked, an object simulating a flock structure must be created and its
       properties set. Afterwards, by calling the "lock()" method a lock can be set or it can be
       determined if and which process currently holds the lock.

       To create a new object representing a flock structure call "new()":

         $fs = new File::FcntlLock;

       You also can pass the "new()" method a set of key-value pairs to initialize the objects
       properties, e.g. use

         $fs = new File::FcntlLock l_type   => F_WRLCK,
                                   l_whence => SEEK_SET,
                                   l_start  => 0,
                                   l_len    => 100;

       if you plan to obtain a write lock for the first 100 bytes of a file.

       Once you have created the object simulating the flock structure the following methods
       allow to query and in most cases also to modify the properties of the object.

       "l_type()"
           If called without an argument returns the current setting of the lock type, otherwise
           the lock type is set to the argument, which must be either "F_RDLCK", "F_WRLCK" or
           "F_UNLCK" (for read lock, write lock or unlock).

       "l_whence()"
           Queries or sets the "l_whence" property of the flock object, determining if the
           "l_start" value is relative to the start of the file, to the current position in the
           file or to the end of the file. The corresponding values are "SEEK_SET", "SEEK_CUR"
           and "SEEK_END". See also the man page for lseek(2).

       "l_start()"
           Queries or sets the start position (offset) of the lock in the file according to the
           mode selected by the "l_whence" member.  See also the man page for lseek(2).

       "l_len()"
           Queries or sets the length of the region (in bytes) in the file to be locked. A value
           of 0 is interpreted as to mean a lock (starting at "l_start") up to the very end of
           the file.

           According to SUSv3 negative values for "l_start" are allowed (resulting in a lock
           ranging from "l_start + l_len" to "l_start - 1") Unfortunately, not all systems allow
           negative arguments and will return an error when you try to obtain the lock, so please
           read the fcntl(2) man page of your system carefully for details.

       "l_pid()"
           This method allows retrieving the PID of a process currently holding the lock after a
           call of "lock()" with "F_SETLK" indicated that another process is holding the lock. A
           call to "lock()" with "F_GETLK" will fill in this value so "l_pid()" can be called.

       When not initialized the flock objects "l_type" property is set to "F_RDLCK" by default,
       "l_whence" to "SEEK_SET", and both "l_start" and "l_len" to 0, i.e. the settings for a
       read lock on the whole file.

       After having set up the object representing a flock structure you can determine the
       current holder of a lock or try to obtain a lock by invoking the "lock()" method with two
       arguments, a file handle (or a file descriptor, the module figures out automatically what
       it got) and a flag indicating the action to be taken, e.g.

         $fs->lock( $fh, F_SETLK );

       There are three values that can be used as the second argument:

       "F_GETLK"
           For "F_GETLK" the "lock()" method determines if and who currently is holding the lock.
           If no other process is holding the lock the "l_type" field is set to "F_UNLCK".
           Otherwise the flock structure object is set to the values that prevent us from
           obtaining a lock.  There may be multiple such blocking processes, including some that
           are themselves blocked waiting to obtain a lock. "F_GETLK" will only make details of
           one of these visible, and one has no control over which process this is.

       "F_SETLK"
           For "F_SETLK" the "lock()" method tries to obtain the lock (when "l_type" is set to
           either "F_WRLCK" or "F_RDLCK") or releases the lock (if "l_type" is set to "F_UNLCK").
           If a lock is held by some other process the method call returns "undef" and errno is
           set to "EACCESS" or "EAGAIN" (please see the the man page for fcntl(2) for the
           details).

       "F_SETLKW"
           is similar to "F_SETLK" but instead of returning an error if the lock can't be
           obtained immediately it blocks until the lock is obtained. If a signal is received
           while waiting for the lock the method returns "undef" and errno is set to "EINTR".

       On success the method returns the string "0 but true". If the method fails (as indicated
       by an "undef" return value) you can either immediately evaluate the error number (usingf
       $!, $ERRNO or $OS_ERROR) or check for it at some later time via the methods discussed
       below.

       There are three methods for obtaining information about the reason the the last call of
       "lock()" for the object failed:

       "lock_errno()"
           Returns the error number from the latest call of "lock()". If the last call did not
           result in an error the method returns "undef".

       "error()"
           Returns a short description of the error that happened during the latest call of
           "lock()" with the object. Please take the messages with a grain of salt, they
           represent what SUSv3 (IEEE 1003.1-2001) and the Linux, TRUE64, OpenBSD3 and Solaris8
           man pages tell what the error numbers mean, there could be differences (and additional
           error numbers) on other systems. If there was no error the method returns "undef".

       "system_error()"
           While the previous method, "error()", tries to return a string with some relevance to
           the locking operation (i.e. "File or segment already locked by other process(es)"
           instead of "Permission denied") this method returns the "normal" system error message
           associated with errno. The method returns "undef" if there was no error.

   EXPORT
       F_GETLK F_SETLK F_SETLKW F_RDLCK F_WRLCK F_UNLCK SEEK_SET SEEK_CUR SEEK_END

CREDITS

       Thanks to Mark Jason Dominus (MJD) and Benjamin Goldberg (GOLDBB) for helpful discussions,
       code examples and encouragement. Glenn Herteg pointed out several problems and also helped
       improve the documentation.  Julian Moreno Patino also helped correcting the documentation
       and pointed out problems arising on GNU Hurd (which seems to have only very rudimentary
       support for locking with fcntl()).

AUTHOR

       Jens Thoms Toerring <jt@toerring.de>

SEE ALSO

       perl(1), fcntl(2), lseek(2).